One of the most difficult network-based attacks to defend against is that of a class known as “man-in-the-middle” (MITM) attacks. These scenarios involve the insertion by an attacker of an untrusted node into a session path. This MITM attack node then impersonates the identity of one session partner to the other while having access to and control over all transmissions between the partners resulting in the potential for eavesdropping and unauthorized modification of session traffic. These attacks are particularly effective when combined with “phishing” techniques which trick an end user into giving up personal information that later results in identity theft or fraud.
FIG. 2 depicts an example of an MITM attack, in which a plurality of users 50 are unsuspectingly engaged in sessions with an MITM Web server 52, which sits between users 50 and the legitimate target Web server 54. In this case, MITM Web server 52 has lured users 50 into believing that they are engaging in a session with the target Web server 54.
Banks and other financial institutions have been particularly hard hit by these attacks as customers are lured into logging onto a malicious MITM node thinking they are connecting to their bank, while the MITM node interacts with the legitimate web site and relays information to the user making the ruse even more believable. It is this last aspect that is particularly distressing to banks as they do not have a good way to detect that they are dealing with an MITM node rather than directly with their customer. Because the attacker's MITM machine looks and acts just like a real end user, the MITM machine acts as a conduit for sensitive information such as userids and passwords and, potentially, for conducting transactions.
While digital certificates could provide a reasonable measure of protection in such an instance, they have thus far proven to be difficult and expensive to deploy and manage across a large end user base especially when those users may have limited technical skills and may also be highly mobile, using many different nodes to conduct transactions.
Another increasingly popular defense is to use “strong” authentication mechanisms such as security tokens, smart cards or biometrics to ensure that the end user is who they claim to be. The problem with these defenses is that in the MITM case, the digitized authentication data is simply passed directly through the intermediate node making it unnecessary for the attacker to actually possess the token, smart card or physical characteristic being measured.
Finally, another technique for alerting end users that they are being lured into logging into a bogus Web site involves the presentation of a graphical image pre-selected by the user. During login, if the user does not see the image they selected during registration (something which only they and the legitimate Web site should know), then they know to abort the authentication process. However, in an MITM case, the target Web site presents the image to the MITM node thinking it is the end user and the MITM node relays this image to the real end user leading them to believe that the attack node is legitimate.